With regard to organic EL devices, particularly, low molecular weight organic EL (hereinafter abbreviated as OLED) devices, ultrathin film formation of organic layer and functional separation based on multilayering have been promoted by Eastman Kodak Co., thereby significantly improving characteristics of the devices such as a remarkable lowering of drive voltage and the like (Non-Patent Document 1: Applied Physics Letters, U.S.A., 1987, Vol. 51, pp. 913-915).
Devices of organic EL making use of a polymeric luminescent material (hereinafter abbreviated as PLED) have been discovered by Cambridge University (Non-Patent Document 2: Nature, the United Kingdom, 1990, Vol. 347, pp. 539-541). The characteristics of the recent polymeric organic EL devices have been improved to a level not far behind from conventional OLED devices.
As to the OLED devices, it has been reported that when a copper phthalocyanine (CuPC) layer is provided as a hole injection layer, a lowering of drive voltage and improvements of initial characteristics such as an luminous efficiency and a life characteristic can be realized (Non-Patent Document 3: Applied Physics Letters, U.S.A., 1996, Vol. 69, pp. 2160-2162).
On the other hand, it has also been reported that effects similar to those of OLED devices can be obtained when using, as a hole transport layer (buffer layer), polyaniline materials (Non-Patent Document 4: Nature, the United Kingdom, 1992, Vol. 357, pp. 477-479, and Non-Patent Document 5: Applied Physics Letters, U.S.A., 1994, Vol. 64, pp. 1245-1247) and polythiophene materials (Non-Patent Document 6: Applied Physics Letters, U.S.A., 1998, Vol. 72, pp. 2660-2662).
Furthermore, it has been found that initial characteristics are improved when using, as an electron injection layer at the cathode side of these devices, metal oxides (Non-Patent Document 7: IEEE Transactions on Electron Devices, U.S.A., 1997, Vol. 44, pp. 1245-1248), metal halides (Non-Patent Document 8: Applied Physics Letters, U.S.A., 1997, Vol. 70, pp. 152-154), metal complexes (Non-Patent Document 9: Japanese Journal of Applied Physics, 1999, Vol. 38, pp. L1348-1350) and the like. These charge injection layers and buffer layers have now been in ordinary use.
In recent years, an organic solvent-based, charge transport varnish making use of a low molecular weight oligoaniline material has been discovered and it has been found that excellent EL device characteristics are shown by inserting a hole injection layer obtained by use of the above material (Patent Document 1: JP-A 2002-151272).
However, CuPC that is a hole injection material ordinarily used in OLED devices is very irregular in shape, with the attendant drawback that if it is incorporated in other organic layers in very small amounts, characteristics are greatly lowered.
The polyaniline and polythiophene materials currently employed in PLED devices involve problems in that they contain, as a solvent, water that has the possibility of promoting device degradation, limitation is placed on the choice of solvent, and limitation is also placed on the coating method ensuring uniform film formation because of the agglomeration and low solubility of the materials.
Moreover, in case where there is used an organic solvent-based, charge transporting varnish containing a low molecular weight oligoaniline material of high solubility, there may arise problems in that limitation is placed on the kind of usable electron-accepting dopant material and the heat resistance and amorphousness of an electron-accepting dopant are low. Additionally, in an electron transport varnish containing a low molecular weight charge transport material and charge-accepting dopant material, particularly a varnish containing a crystalline material, a difficulty may generally be involved in film formation ensuring high flatness.                Patent Document 1: JP-A 2002-151272        Patent Document 2: WO 2006/025342        Non-Patent Document 1: Applied Physics Letters, U.S.A., 1987, Vol. 51, pp. 913-915        Non-Patent Document 2: Nature, the UK, 1990, Vol. 347, pp. 539-541        Non-Patent Document 3: Applied Physics Letters, U.S.A., 1996, Vol. 69, pp. 2160-2162        Non-Patent Document 4: Nature, the UK, 1992, Vol. 357, pp. 477-479        Non-Patent Document 5: Applied Physics Letters, U.S.A., 1994, Vol. 64, pp. 1245-1247        Non-Patent Document 6: Applied Physics Letters, U.S.A., 1998, Vol. 72, pp. 2660-2662        Non-Patent Document 7: IEEE Transactions on Electron Devices, U.S.A., 1997, Vol. 44, pp. 1245-1248        Non-Patent Document 8: Applied Physics Letters, U.S.A., 1997, Vol. 70, pp. 152-154        Non-Patent Document 9: Japanese Journal of Applied Physics, 1999, Vol. 38, pp. L1348-1350        